1. Technical Field of the Invention
The present invention relates to an apparatus and to a method for producing pellets by layering powder onto particles.
The apparatus comprises a vessel and a rotor having a supporting surface on its upper side for supporting particles. The rotor is arranged in said vessel and supported for rotation around a vertical rotational axis. The vessel contains a processing space adjacent to the upper side of said rotor and, thus, to said supporting surface. The apparatus comprises furthermore feeding means for introducing a powder and a liquid into said processing space.
For producing pellets, particles serving to constitute the cores of the pellets to be produced are introduced into the processing space. These particles are designated in the following also as core particles. The rotor is then rotated so that a bed of moving particles is formed on the supporting surface of the rotor. Moreover, a gas--normally air--may be conveyed in an upward direction through an annular gap between a wall portion of the vessel and the rotor. In a layering operation, liquid and powder are introduced into the processing space, atomized and sprayed onto the core particles. The liquid serves thereby to bind the fine powder particles onto the surfaces of the core particles and on powder already layered onto the core particles. The core particles that are originally nude--i.e. comprise no layer--are then provided with a growing powder layer. In this way, one can produce pellets with core particles coated with an envelope comprising powder, wherein the volume and mass and weight of this envelope may be larger, for instance up to five times or even more times larger than the volume and mass and weight, respectively, of the core particles brought originally into the processing space and forming the cores of the pellets.
The produced pellets may serve, for example, as particulate drugs or components of drugs, wherein the envelope formed by powder may comprise at least one pharmaceutical effective or active substance. The produced pellets, may, however, also serve for other purposes and form for instance nutrients, agricultural and other chemicals or seed pellets.
2. The State of the Art
U.S. Pat. No. 4,895,733 discloses apparatuses that comprise a vessel and a disc-type rotor arranged rotatably around a vertical axis in said vessel. A filter is disposed inside the vessel above the rotor. The portion of the interior of the vessel arranged between the rotor and the filter serves as processing space. The apparatuses comprise furthermore a sucking device for sucking air underneath the rotor into the vessel and in an upward direction through an annular gap between the wall of the vessel and the rotor, through the processing space and through the filter. The apparatus comprises moreover feeding means that comprise an introducing member with one or several mouths opening into the processing space for introducing a dry particulate material into the processing space. The feeding means comprise also a plurality of nozzles for introducing a liquid into the processing space.
The apparatuses disclosed in U.S. Pat. No. 4,895,733 are primarily designated to agglomerate particles and are suited well for this purpose. However, these apparatuses may also be used for coating first particles--i.e. core particles--for instance by layering second, smaller particles--i.e. powder--onto said core particles. For this purpose, a batch of core particles is introduced into the processing space and onto the rotor. While the particles are then alternately moved over the rotating rotor and whirled up by air streaming upward through the vessel, liquid is introduced into the portion of the processing space above the bed formed by the particles moving over the rotor. The powder formed by the second particles may be introduced into the processing space, for instance, in dry state continuously or intermittently during the moving of the core particles (see column 3, line 64 to column 4, line 6).
The movements of the nude core particles and of the core particles comprising already a powder layer caused by the rotation rotor and the air sucked through the vessel are rather complicated and in particular different for the individual particles. For this reason and because the powder and liquid are introduced into the processing space above said bed at places that are relatively far away from each other, powder may reach particles that are rather irregularly and differently wetted by the liquid. More specifically, the powder may come into contact in part with more or less dry particles and in part with very wet particles. However, the powder will not adhere to dry particles. Furthermore, particles that are very wet may tend to agglommerate. Thus, the product produced will likely comprise pellets with very different sizes and possibly even irregular forms. Pellets with irregular sizes and possibly irregular forms are, however, for many purposes and particularly for drugs very disadvantageous. The individual pellets comprise then namely different amounts of the pharmaceutical effective substance. When such pellets will be administered for instance orally to a person ar an animal, the different sizes will also result in different delivery rates of the effective substance. The fact that the particles coming into contact with powder are irregularly wet, results also in the disadvantage that a relatively large amount of liquid is needed for wetting the particles so that the time and energy for drying the particles will be relatively high. Furthermore, the air passed through the processing space may lift a considerable fraction of the supplied powder to the filter. As the powder may comprise at least one very expensive pharmaceutical substance, a loss of the powder carried to the filter might considerably increase the costs of the produced pellets. In practice, one may shake the filter for recuperating the powder hanging on the filter. However, it will render the production of pellets more difficult and influence the quality of the pellets unfavorably if very large amounts of powder must be shaken off the filter. Moreover, liquid may possibly be carried to the filter, too. Such liquid may possibly cause the powder hanging on the filter to agglomerate. An agglomeration of the powder carried to the filter may render it difficult or even impossible to reuse said powder and may also obstruct the filter.
There is also mentioned in the cited U.S. Pat. No. 4,895,733 to introduce powder suspended in a liquid that is sprayed into the processing space (see column 4, lines 6 to 11). Coating particles with a dispersion comprising a liquid and a suspended powder has, however, the disadvantage that the dispersion must have a very high content of liquid in relation to its powder content and that the drying of the coated particles requires then a lot of time.